Simple refrigeration systems use a method called vapor compression cycle. The vapor compression cycle is a method in which a compressor such as a piston compressor compresses a low-pressure refrigerant vapor. In a next stage, a condenser condenses the warm compressed vapor, resulting in a partial or complete condensation of the vapor. This condensed refrigerant then passes through a fine capillary tube or other constriction into a larger chamber at low pressure. As the refrigerant enters the larger chamber, it evaporates and absorbs heat, resulting in the vapor. This refrigerant vapor is then routed to the intake of the compressor, thus closing the cycle. This is the so-called closed loop refrigeration system.
A single stage refrigerant system is used to achieve temperatures up to −40° C. For temperatures in the range −50° C. to −80° C. a two-stage cascaded vapor compression system has been proposed. This method uses a single refrigerant per stage and two compressors, one for each stage. For still lower refrigeration temperatures, more cascaded stages are required. Typically, four stage cascade vapor compression systems are proposed for reaching cryogenic temperatures, lower than −150 C.
An advance in technology has been achieved by using a single stage compression system with a mixture of refrigerants. This method has been used to achieve temperatures far below those that can be achieved using a cascaded multi-stage system, for example, the range −100° C. to −210° C. This method uses a mixture of several refrigerants each of which with different boiling points.
In mixed gas refrigeration systems, the refrigerant in gaseous form is compressed in a compressor and thereafter cooled to remove the heat of compression to near-ambient temperatures. Partial condensation of the refrigerant may occur in some cases. The refrigerant is then passed through the high pressure side of a heat exchanger, which is arranged in a cryostat. The high pressure refrigerant leaving the heat exchanger is throttled to a lower pressure in a throttling device such as an orifice or a capillary tube. The refrigerant is then passed through an evaporator where it absorbs heat from the substance (solid or fluid) being cooled.
Researchers have proposed several refrigerant compositions. There is still a need for a refrigerant composition that can achieve better efficiency or a greater cooling effect or both.
Soviet Union patent 627,154 has suggested the composition of the refrigerant as follows:
Nitrogen: 25-40 molar percent
Methane: 20-25 molar percent
Ethane: 15-35 molar percent
Propane: 25-45 molar percent
UK patent 1,336,892 has suggested the composition of the refrigerant as follows:
Nitrogen: 20-70 molar percent
Methane: 10-30 molar percent
Ethane: 10-25 molar percent
Propane: 10-25 molar percent
U.S. Pat. No. 5,441,658 has suggested the composition of the refrigerant as follows:
Nitrogen: 30-50 molar percent
Methane: less than 20 molar percent
Propane: greater than 30 molar percent
Ethane: balance
This specification describes the optimum refrigerant composition to be used in a single stage refrigeration cycle.